1. Field of the Invention
This invention relates to a method of logging a subsurface formation penetrated by a wellbore to determine movable oil saturation. It particularly concerns a method to determine movable oil saturations in formations where the salinity and capture cross section of the formation water is unknown.
2. Setting of the Invention
In planning or engineering the production of oil from an underground formation which has been subjected to a waterflood, it is important to know the amount of oil in place. The oil in such underground reservoirs is contained in pores of the rock. However, these pores contain more than oil. It is known that all such pores contain some water and quite frequently also contain gas. It is most important to know the amount of movable oil contained in the rock reservoir to evaluate production, secondary recovery and reservoir completion methods.
Cores can be recovered to the surface and the rock porosity determined; however, the knowledge of the porosity in the rock does not give a complete knowledge of the amount of the oil in the rock. One must still determine what part of the core pore space is filled with water, with oil and with gas. One can measure the quantities of gas, oil and water in the core that have been brought to the surface and determined the water, oil and gas saturations in the core. However, experience has shown that the fluid content of the core at the surface is seldom the same as the fluid content of the core in its natural condition in a reservoir. The only logging device available which is described as capable of measuring formation oil saturation or movable oil saturation directly is a carbon oxygen log which does not attain sufficient measurement accuracy. Formation oil saturation means that percent of the fluid in the pore space of the reservoir rock that is oil and movable oil saturation means the difference between the total oil presently in place and the residual oil saturation left after a waterflood technique has been performed on a formation.
There are logging devices, however, which can provide a measure of water saturation. Water saturations can be computed from these logging devices with an accuracy of .+-.15%. If gas saturation exists, it can be measured by independent methods such as described in U.S. Pat. No. 3,282,095.
Oil saturation may be calculated as the difference between pore space calculated at the surface from the core and the water and gas saturations. Such calculations are illustrated by U.S. Pat. No. 3,825,752, Murphy, et al., entitled "Log-Inject-Log" System and issued July 23, 1974.
Movable oil saturations may be calculated for formations in which the capture cross section of the formation water is known. Methods for such calculations are illustrated in Murphy, R. P. and Owens, W. W., "The Use of Special Coring and Logging Procedures for Defining Reservoir Residual Oil Saturations," Journal of Petroleum Technology (July 1973) and Murphy, R. P., Foster, G. T., and Owens, W. W., "Evaluation of Waterflood Residual Oil Saturations Using Log-Inject-Log Procedures," SPE Paper 5804, presented at SPE-AIME Symposium, March 1976.
Most reservoirs which have been subjected to waterflood and some primary reservoirs contain waters of multiple levels of salinity. There are no methods using log-inject-log or conventional techniques presently available for determining movable oil saturations in a reservoir containing multiple or varying salinity waters.